1. Field of the Invention
The present invention relates generally to the field of radio frequency (RF) power supplies. The present invention is useful in induction heating and plasma applications, but by no means is limited to such applications.
2. Discussion of the Background
Induction heating is a method of heating electrically conductive materials such as metals. Induction heating relies on, as the name implies, inducing electrical currents within a material to be heated. These induced currents, called eddy currents, dissipate energy and bring about heating. Common uses of induction heating include heat treating, welding, melting, packaging and curing. The number of consumer items which undergo induction heating during some stage of their production is large and rapidly expanding.
Prior to the development of induction heating, gas and oil-fired furnaces provided the prime means of heating metals and nonmetals. The advantages that induction heating offers over furnace techniques are numerous. For example, greater heating rates can be achieved by induction heating than can be achieved by gas or oil furnaces. Higher heating rates lead to shorter heating times, which lead to productivity increases and reduced labor costs. Furthermore, given today's environmental concerns, induction heating is an attractive alternative to pollution producing furnaces.
The basic components of an induction heating system are (1) an AC power source (RF) power supply, (2) a tank circuit having an inductor coil and a capacitor, and (3) the material to be heated (a.k.a., "workpiece" or "load"). Common tank circuits used in induction heating are either parallel resonant or series resonant. A parallel resonant tank circuit includes a capacitance in parallel with the inductor coil and a series resonant tank circuit includes a capacitance in series with the inductor coil. A workpiece is heated by placing the workpiece within the inductor coil of the tank circuit and applying a high-power, RF alternating voltage to the tank circuit using the power supply. (The alternating voltage applied to the tank circuit causes an alternating current to flow through the inductor coil. The flow of an alternating current through the inductor coil generates an alternating magnetic field that cuts through the workpiece placed in the inductor coil. It is this alternating magnetic field that induces the eddy currents that heat the workpiece.)
A workpiece is heated most efficiently when the frequency of the alternating voltage applied to the tank circuit matches the tank circuit's resonant frequency. That is, when the tank circuit (i.e., the tank circuit with a workpiece placed in the inductor coil) is driven at its resonant frequency, the transfer of power from the power supply to the workpiece is maximized. Thus, heating of the workpiece at the resonant frequency yields the greatest heating efficiency.
It should be noted that the resonant frequency of the tank circuit is in part determined by the characteristics of the inductor coil, such as the size and shape of the coil, and the characteristics of the workpiece when the workpiece is placed in the coil. Hence, moving the workpiece through the coil or altering the characteristics of the workpiece by heating it will change the resonant frequency of the tank circuit. Because the resonant frequency of the tank circuit changes as the workpiece is heated or moved through the coil, induction heating systems utilize a power supply having a tuning system for continuously tracking the resonant frequency of the tank circuit. By tracking the resonant frequency of the tank circuit, the power supply is better able to provide an alternating voltage that matches the resonant frequency, thereby efficiently heating the workpiece.
A problem with conventional induction power supplies, however, is that they operate over a limited frequency band. Another problem is that they are not capable of delivering a power into a load that is remotely located from the power supply. Therefore, what is desired is an RF power supply that overcomes the above and other limitations of conventional RF power supplies.